A truss is a structure consisting of straight members connected at their extremities only. The members being slender and unable to support lateral loads, all the loads must be applied at the joints; a truss may thus be assumed to consist of pins and two-force members.

two-force member pinpin

Chapter 6 ANALYSIS OF STRUCTURES

A truss is said to be rigid if it is designed in such a way that it will not greatly deform or collapse under a small load. A triangular truss consisting of three members connected at three joints is clearly a rigid truss.

A

B

C

A truss obtained by adding two new members to the first one and connecting them to a new joint (D ) will also be rigid. Trusses obtained by repeating this procedure are called simple trusses. We may check that in a simple truss the total number of members is m = 2n - 3, where n is the total number of joints.

A

B

C A

B

C

D

A

B

C

The forces in the various members of atruss can be determined by the methodof joints. First, the reactions at the supportscan be obtained by considering the entiretruss as a free body. The free-body diagram of each pin is then drawn, showingthe forces exerted on the pin by the membersor supports it connects. Since the members are straight two-force members, the force exerted by a member on the pin is directed along that member, and only the magnitude of the force is unknown. It is always possible in the case of a simple truss todraw the free-body diagrams of the pins in such an order that onlytwo unknown forces are included in each diagram. These forces can be obtained from the corresponding two equilibrium equations or - if only three forces are involved - the correspondingforce triangle.

A

B

C

Tension (T)

Compression (C)

C

T T

C

If the force exerted by a member on a pin is directed toward that pin, the member is in compression ; if it is directed away from the pin, the member is in tension. The analysis of a truss is sometimes expedited by first recognizing joints under special loading conditions (involving zero-force members, for example). The method of joints can also be extended to the analysis of three-dimensional or space trusses.

The method of sections is usually preferred to the method of joints when the force in only one member - or very few members - of a truss is desired. To determine the force in member BD of the truss shown, we pass a section through members BD, BE, and CE, remove these members, and use the portion ABC of the truss as a free body.

AB

C

D

E

G

P1 P2 P3

n

n

AB

C

D

E

G

P1 P2 P3

n

n

Writing ME = 0, we determine the magnitude of FBD, which represents the force in member BD. A positive sign indicates that the member is in tension; a negative sign indicates that it is in compression.

AB

C E

P1 P2

FBD

FBE

FCE

B

C E

P1 P2

FBD

FBE

FCE

The method of sections is particularly useful in the analysis ofcompound trusses (trusses which cannot be constructedfrom the basic triangular truss, but which can be obtained by rigidly connecting several simple trusses). If the componenttrusses have been properly connected (e.g., one pin and onelink, or three nonconcurrent and nonparallel links) and if theresulting structure is properly supported (e.g., one pin and oneroller), the compound truss is statically determinate, rigid, andcompletely constrained. The following necessary - but not sufficient - condition is then satisfied: m + r = 2n, where m is the number of members, r is the number of unknowns representing the reactions at thesupports, and n is the number of joints.

Frames and machines are structures which contain multiforce members, i.e., members acted upon by three or more forces. Frames are designed to support loads and are usually stationary, fully constrained structures.

A B C D

M

Machines are designed to transmit or modify forces and always contain moving parts.

To analyze a frame, we first consider the entire frame as a free body andwrite three equilibrium equations. If the frame remains rigid when detached from its supports, the reactions involve only three unknowns and may be determined from these equations. On the other hand, if the frame ceases to be rigid when detached from its supports, the reactions involve more than three unknowns and cannot be completely determined from the equilibrium equations of the frame.

A B C D

M

EyEx

Dy

Dx

We draw the free-body diagram of each of the multiforce members, noting that when two multiforce members are connected to the same two-force member, they are acted upon by that member with equal and opposite forces of unknown magnitude but known direction. When two multiforce members are connected by a pin, they exert on each other equal and opposite forces of unknown direction, which should be represented by two unknown components.

Then dismember the frame and identify the members as either two-force members or multiforce members; pins are assumed to form an integral part of one of the members they connect.

AB

C D

M

EyEx

Dy

Dx

BB

B

The equilibrium equations obtained from the free-body diagrams of the multiforce members can then be solved for the various internal forces. The equilibrium equations can also be used to complete the determination of the reactions at the supports. Actually, if the frame is statically determinate and rigid, the free-body diagrams of the multiforce members could provide as many equations as there are unknown forces. However, as suggested above, it is advisable to first consider the free-body diagram of the entire frame to minimize the number of equations that must be solved simultaneously.

To analyze a machine, we dismember it and, following the same procedure as for the frame, draw the free-body diagram of each of the multiforce members. The corresponding equilibrium equations yield the output forces exerted by the machine in terms of the input forces applied to it, as well as the internal forces at various connections.